This invention relates to a length measuring device for measuring the relative position of first and second objects, of the type comprising a measuring scale which defines a measuring direction, means for connecting the scale to the first object, a scanning unit, and means for guiding the scanning unit along the scale independently of movement of the second object transverse to the scale. In particular, this invention relates to an improved coupling arrangement for such length measuring devices.
Such length measuring devices include a scanning unit that is guided by an auxiliary guide means, independently of the guidance of the object whose position is being measured, and require an articulated coupling of the scanning unit to the object being measured or to a coupling member mounted to it.
When the scale is scanned by the scanning unit, the coupling can result in measurement errors if the auxiliary guide for the scanning unit does not run parallel to the guidance of the object to be measured, or if there arise direction changes between the auxiliary guide for the scanning unit and the center line of the division of the cable. Measurement errors which result from direction changes between the auxiliary guide for the scanning unit and the center line of the division (direction errors) become negligibly small if the scanning unit is coupled to the object being measured in the neutral fiber of the scale. In this specification and the following claims, the term "neutral fiber" is used to refer to the line of intersection between the neutral plane of the scale and the plane which runs through the center line of the division of the scale perpendicularly to the neutral plane of the scale.
Measurement errors which result from a lack of parallelism between the auxiliary guide of the scanning unit and the guide of the object to be measured become negligibly small only if the coupling arrangement is constructed in such a way that it permits relative movements between the scanning unit and the object being measured in a plane perpendicular to the measurement direction, while eliminating all changes in length in the measuring direction.
In German Pat. No. 28 10 341 there is described a length measuring instrument in which the scanning unit is coupled by means of a coupling element to a follower. In this patent, the coupling element takes the form of a wire which is rigid in the measuring direction and is positioned near the neutral fiber of the scale. Deflections of the wire caused by lack of parallelism between the guide of the object to be measured and the auxiliary guide of the scanning unit result in length changes of the wire in the measurement direction. Such length changes, of course, result in measurement errors. Furthermore, since the wire does not lie in the neutral fiber of the scale, direction errors of the type described above also result in measurement errors.
German DE-OS No. 25 05 586 discloses a length measuring instrument which also includes a scanning unit that is coupled to a follower by means of a rigid wire. Simultaneously, this wire biases the scanning unit onto and against the auxiliary guide. As discussed above, this wire changes the distance between the scanning unit and the follower in response to changes in parallel movements, and in response to direction errors the pivot point does not lie in the neutral fiber of the scale. Thus, measurement errors through lack of parallelism and through direction errors are produced.
In German Pat. No. 26 11 459 there is shown a length measuring instrument in which a scanning unit is coupled in an articulated manner to the follower by means of a coupling. The coupling includes a plane surface on the scanning unit which is held in constant contact with a spherical surface of the follower by means of a spring. In this coupling arrangement, the coupling point lies outside the neutral fiber of the scale, and the distance between the coupling point and the neutral fiber is subject to fluctuations through parallel shifting between the auxiliary guide and the guide of the object to be measured, so that measurement errors can arise in consequence of direction errors.
German DE-AS No. 2 219 624 relates to a measuring instrument in which the scanning unit is coupled in an articulated manner to the follower by means of a wire which is rigid in the measuring direction. This wire is connected with a quidrilateral link, the momentary pole of which lies in the neutral fiber of the scale. With this coupling arrangement, measuring errors resulting from direction errors are avoided. However, measurement errors can result through deflections of the wire in consequence of a lack of parallelism.
German Pat. No. 25 10 219 discloses a length measuring instrument in which a scanning unit is coupled to a follower in an articulated manner. This coupling includes a first leaf spring positioned in the plane of and at the height of the neutral plane of the scale, and a second leaf spring in a plane perpendicular to the neutral plane of the scale passing through the neutral fiber of the scale. This coupling arrangement succeeds in eliminating measurement errors which might otherwise result from direction errors in guidance. However, deflections of the leaf springs in consequence of a lack of parallelism can still result in measurement errors.